New developments in two-feldspar thermometry

Abstract

The thermodynamic model of the two-feldspar thermometer has been revised. From recent enthalpy and volume measurements in the (Na,Ca)-and (K,Ca)-feldspar binaries, new interaction parameters have been derived and previous ones have been updated. Entropy parameters have been fi tted to the phase equilibrium data of Seck (1971) and Elkins and Grove (1990). The two data sets could be suitably combined into one. Ideal Ab, Or, and An activities have been expressed in terms of both the molecular mixing and Al-avoidance models.Two-feldspar pairs from high-grade metamorphic rocks that cooled slowly under dry conditions suffer from a distinct type of retrograde resetting. Whereas the original An content in both the plagioclase and the alkali feldspar is preserved because the intercrystalline Ca + Al ↔ (Na,K) + Si diffusion is sluggish, Na and K may be freely exchanged between phases. Mathematical reversal of the Na-K exchange at constant An yields the temperature at which the two feldspars originally coexisted. The shifts in Ab and Or contents obtained from the reversal refl ect the relative plagioclase/alkali feldspar proportions observed in thin sections. Good agreement between calculated and measured ratios was found for feldspar pairs from Sri Lankan granulites. This observation represents a successful test of the reliability of the calculated Ab-Or shifts.In contrast to dry metamorphic rocks, similar application of chemical constraints is not indicated in the case of volcanic rocks. Then the two-feldspar thermometer delivers three, usually incongruent temperatures: T(Ab), T(Or), and T(An). From the abundance of temperatures, Fuhrman and Lindsley (1988) suggested adjusting compositions within assumed chemical uncertainties (e.g., ±2 mol%) so that congruent temperatures could be obtained. However, the result is not unique. Depending on minute variations in the starting compositions, the temperatures may vary by several tens of degrees. In addition, temperatures vary to a similar extent depending on the type of search algorithm. Therefore, we advise users to completely abandon this practice. Instead, a statistical procedure is suggested: Two-feldspar compositions are randomly generated according to Gaussian distributions with their means at the observed compositions and standard errors chosen according to the quality of the chemical analysis. This procedure returns normally distributed temperatures [T(Ab), T(Or), T(An)] together with means and standard deviations. From the overlap of the three Gaussian curves the question of equilibrium or non-equilibrium crystallization of feldspar pairs may be addressed.

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

“…We used the Zr-in-rutile thermometer calibration of Tomkins et al (2007) (Table 1). For ternary feldspar thermometry, antiperthite compositions were reintegrated following ; the T estimates used the feldspar activity models of Benisek et al (2004Benisek et al ( , 2010. The method estimates the minimum T for the crystallization of homogeneous ternary feldspar.…”

“…The temperature estimates for antiperthites from samples 167 and 172 range from 921-1005 °C at 1 GPa using the feldspar activity models of Benisek et al (2004Benisek et al ( , 2010. The antiperthites record UHT conditions regardless of the feldspar activity model used (Table 6).…”

“…Here we provide new temperature estimates for the adjacent Upper Member of the Bigelow Brook Formation. Ternary feldspar reintegration using the activity model of Benisek et al (2004) and Zr-in-rutile thermometry (Tomkins et al 2007) give average temperatures of ~990 and ~1010 °C, respectively, at 1 GPa for this unit. Therefore, the recently discovered UHT zone in the Brimfield Schist of northeastern Connecticut extends to the east to include the Upper Member of the Bigelow Brook Formation.…”

Petrological and Sr–Nd–Os isotopic constraints on the origin of the Fanshan ultrapotassic complex from the North China Craton

“…The occurrence of melanites and the pure orthoclase compositions of feldspars argue for the high temperature features of the magma (probably >1000°C; Benisek et al, 2004). In addition, the magma must have high oxygen fugacity, as inferred from the ubiquitous Boynton (1984), and the primitive mantle values are from Sun and McDonough (1989).…”

Fusão por desidratação de quartzo-biotita-anfibolito do Complexo Piracaia (Estado de São Paulo) sob temperatura de 950º C e pressões de 900 e 1100 MPa e implicações para a geração de magmas graníticos

“…. Diagrama Ab-An-Or com isotermas de 800, 900 e 100 o C, para uma pressão de 1000 MPa (BENISEK et al, 2004). Círculos fechados: composições obtidas no experimento sob 1100 MPa; abertos, experimento de 900 MPa.…”